Put down those oranges, lemons are a better natural source.

More than 135,000 cases of the flu, with four active strains, have been recorded so far in 2017, although the true figure is likely much higher as many cases go unreported.

One of the most common remedies for illness is vitamin C. While it's known as a powerful antioxidant and metabolism booster, there is no concrete research telling us how much we need to restore ourselves to health.

"Vitamin C has been linked to immunity, the immune system [and] vitamin C has been linked to the common cold," Ronda Greaves, Senior Lecturer in Clinical Biochemistry at RMIT University, told HuffPost Australia.

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Vitamin C has been linked to immunity and the common cold.

"Vitamin C is a vitamin, and a vitamin means it's a nutrient ... that is essential to our diet that we can't produce ourselves.

"We need vitamin C, end of story ... we can't produce it, so we have to have it. How much we need to have is still up for debate when people are sick," Greaves said.

"There's been studies looking at megadoses of vitamin C with the common cold, but people sometimes look at what that dose is and not at what is really in the body."

The problem with determining how much vitamin C people need to recover from sickness comes down to the unstable nature of the vitamin, which ultimately makes it hard to measure.

"It's difficult because vitamin C is not stable," Greaves said. "So when you take it [vitamin C] out -- by actually drawing the blood -- the vitamin C will actually just disappear if you just leave it sitting at room temperature.

"Because of the difficulties of the stability of vitamin C itself, I think there are a lot of confounders in studies and that is really why we don't know as much as we probably could."

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Researchers don't know how much vitamin C a person needs to recover from illness.

What researchers do know however, is how much vitamin C we need to maintain desirable health levels.

Vitamin C is well known for its ability to regenerate tissues and aid in the absorption of elements like iron. It's also a powerful antioxidant and helps the body in forming and maintaining things like blood vessels and skin and because we can't produce it, we need to get it from our food.

Greaves says that citrus fruits are the way to go.

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Lemons are one of the best sources of vitamin C.

"Oranges, lemons, limes, the citrus fruits [are good], but lemons are actually the best source."

Other natural sources include red capsicum, kale, broccoli, strawberries and Brussels sprouts. While this is still a developing research area, the good news is, there are no known side affects of having a little extra vitamin C if you are in the mood.

"It is a water-loving vitamin, it likes to be in the water and it is easily excreted in the urine, so I think any excess just goes out."